The invention relates to a rotor shaft for a rotary machine, which rotor shaft has bearing parts which, after being fitted in the machine, are held in oil-lubricated sliding-contact bearings.
Rotor shafts of this kind are known in rotary machines, such as steam turbines, gas turbines, centrifugal pumps, boiler feed pumps, axial compressors and centrifugal compressors. Rotary machines of this kind frequently operate at high rotational speeds, which in the case, for example, of centrifugal compressors may lie between 3000 and 18,000 revolutions per minute.
A problem which may arise in rotary machines where the rotor shaft rotates at such high speeds is synchronous instability of the rotor shaft, leading the machine to vibrate. The cause of this synchronous instability of the rotor shaft is that at relatively high speeds of the rotor shaft a temperature difference is produced diametrically across the shaft at the location of the bearings. Such a temperature difference causes the rotor shaft to bend slightly at the location of the bearing, resulting in imbalance and consequently vibrations. This process can amplify itself, with the result that synchronous rotor instability may arise.
This phenomenon has been investigated by F. M. de Jongh and P. G. Morton and has been described in ASME article 94-GT-35: xe2x80x9cThe synchronous instability of a compressor rotor due to bearing journal differential heatingxe2x80x9d.
Applicant has carried out measurements on a rotor shaft supported directly in bearing cups according to the state of the art. At the location of the bearing an average shaft temperature of about 50 to 70xc2x0 C. and a diametrical temperature difference over the shaft of about 10 to 15xc2x0 C. was measured.
The object of the invention is to eliminate the problem of temperature-induced instability and the resultant vibrations of the rotor shaft in rotary machines.
This object is achieved according to the invention by the fact that the rotor shaft is provided, in the region of at least one bearing part, with an outer casing which forms a thermally insulating layer between the outer surface of the rotor shaft and that part of the shaft which is situated inside the outer casing, said outer casing comprising one single, separate, cylindrical sleeve arranged on the shaft part situated inside the outer casing and forming a structural unity therewith, said sleeve having a cylindrical outer surface which forms, in the region of the bearing part, the outer surface of the rotor shaft.
This measure has been found to eliminate the problem of temperature-induced bending of the rotor shaft, with the result that the problem of temperature-induced instability of the rotor shaft no longer occurs.
It is known per se to provide a rotor shaft, at the location of a bearing, with a thermally insulating layer (see for example DE-U-8706954 and DE-A-3734524). However, these constructions are often complicated and, moreover, have a totally different purpose than the construction according to the invention.
The invention also relates to a rotary machine provided with a rotor shaft according to the invention.
The invention will be explained in the following description of an exemplary embodiment with reference to the drawing.